1. Field of the Invention
The present invention relates to control circuits, and particularly, to a control circuit having a multi-function terminal.
2. Description of the Prior Art
Please refer to FIG. 1, which is a diagram of a flyback converter 10. After filtering and current rectification, an input voltage Vin is applied to a primary-side winding Lp of a transformer TX. A power switch 101 is controlled by a pulse width modulation (PWM) control signal Vg. A sense resistor Rs senses a primary-side current Ip flowing through the power switch 101. A control circuit 100 determines duty cycle of the control signal Vg according to a feedback signal Vcomp, which corresponds to an output terminal of the converter 10, and the primary-side current Ip, thereby stabilizing an output voltage Vo.
As is well-known in the art, and shown in FIG. 1, the output voltage Vo on a capacitor Co is generated by rectifying the current through the secondary-side winding of the transformer Tx and a diode D. The feedback compensation signal Vcomp may be generated by using a feedback module 102 to sense the amplitude of the output voltage Vo. Feedback module 102 may include a voltage divider, an optical-coupler, or other device as illustrated in FIG. 1.
Resistors 130 and 140 form a voltage divider. Control circuit 100 utilizes the voltage divider to sense a sense voltage VBNO. Thus, level of the sense voltage VBNO is proportional to the input voltage Vin. When sense voltage VBNO is lower than a predetermined level, control circuit 100 disables switching of power switch 101, so as to prevent converter 10 from operating at too low of an input voltage Vin, which would damage the converter 10.